Serial Attached SCSI (SAS) is a term referring to various technologies designed to implement data transfer between computer devices. The SAS protocol is a serial successor to the parallel Small Computer System Interface. In the SAS protocol, all SAS devices are either an initiator device, a target device, or an expander device. Initiator devices are devices that begin a SAS data transfer, while target devices are the devices to which initiator devices transfer data. Together, initiator devices and target devices are also known in some cases as end devices.
SAS expanders are devices that facilitate data transfer between multiple initiator devices and multiple target devices. The SAS protocol utilizes a point-to-point bus topology. Therefore, if an initiator device is required to connect to multiple target devices, a direct connection must be made between the initiator device and each individual target device in order to facilitate each individual data transfer between the initiator device and each individual target device. SAS expanders manage the connections and data transfer between multiple initiator devices and multiple target devices. SAS expanders may contain SAS devices.
In a typical SAS topology when a device goes missing, all the SAS devices capable of SMP initiator properties connected to the missing device directly or indirectly would start the discovery. After the completion of discovery, the SAS initiator would update its device table to reflect the modified topology. If the SAS topology is huge with tens of expanders and if the device attached to the lowest expander goes missing, then all the expanders from initiator to the expander where the device went missing will start the discovery. This generates a lot of traffic flow in the topology. Also, Discovery is the highest priority task carried out by SAS initiator so all the I/Os to other devices will be stalled till the discovery process is completed.